During wafer fabrication of solid-state devices, photomasks, and reticles are used to form patterns for each layer of the devices on a semiconductor wafer. Even an extremely small defect created during this process can greatly affect the production yield, circuit reliability, or circuit functionality. Wafers are commonly inspected during each step of the fabrication process to detect such defects. Defects may be correctable requiring additional time and expense before the fabrication process continues. Other defects may require scrapping the entire wafer.
Defects can occur in the resist development process. When resist is exposed, a chemical reaction occurs, which results in the production of various chemical by-products. Some of these by-products are not readily soluble in the developing solution, producing a residue that adheres to the wafer surface. Thus, the residue can be difficult to remove and creates defects. Such by-products tend to adhere to areas of the wafer with either relatively fewer patterns or to areas with large simply geometric patterns. Photolithography resist residue causes such defects and tends to adhere to such areas.
Residue defects adhering to the substrate in a large developed area can interfere with subsequent fabrication steps including the etch and implantation processes. Such interference can affect the performance or operation of the device produced by the process. The residue defect can act as a partial mask causing incomplete etching of the underlying film. This leads to a defect that is subsequently covered by the film when the next layer is applied during the manufacturing process. If this film is non-opaque, the defect will be detected during subsequent defect inspections since the defect can be detected through optically transparent films. The residue defect adds to the resist thickness but still prevents etching or implantation locally. Once the etch or implantation process is complete, the defect is removed with the rest of the resist. Thus, a residue defect that adheres to a resist structure will prevent a decrease in the signal-to-noise ratio at subsequent defect inspections, relative to a residue defect that adheres to the substrate.
When the residue defect adheres to the a resist structure, it is less likely to interfere with subsequent etch or implantation processes. However, such a defect will decrease the signal-to-noise ratio for the defect inspection at the photolithography layer.
"Lithography" is a process of transferring a pattern or image from one medium to another, as from a mask to a wafer. Photolithography is a lithographic technique involving light as the pattern transfer medium.
A "mask" is a transparent, for example glass or quartz, plate covered with patterns used in making integrated circuits. Each pattern consists of opaque and transparent areas that define the size and shape of all circuit and device elements. The mask is used to expose selected areas of resist, which defines areas to be etched. The term "reticle" includes mask, especially a photomask, used in wafer fabrication.
A "photoresist" is a light-sensitive liquid that is spread as a uniform thin film on a wafer or substrate. After baking to solidify the liquid, exposure of specific patterns is performed using a photomask. Material remaining after development shields regions of the wafer from subsequent etch or implant operations.